screw-cutting basics

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Fergus OMore:
GHT gave a full construction article on the VDH in the Workshop Techniques book and much on the small rotary table in Model Engineers Workshop Manual. As I wrote to Minerva- I have built many of the tools and as an amateur found little criticism of the notes. My problem was initially lack of experience- and lack of GHT tooling.

minerva:
It goes without saying that both of GHT books are in my possession and in all honesty I find them rather confusing at times not so much from the technical content but more from the editing point of view. Given that the book was almost certainly composed from magazine articles they have a tendency to be chronologically muddled. For example the first nine pages of chapter 17 describe the initial machining operation in some detail, page ten then informs that if mounted the other way round the setting up is halved and the chances of a more accurate result increased. As to the machining of the threaded parts of the spindle and associated nuts ,his advice Nuts first but no info on bore dia which still makes it a somewhat empirical approach. (suck it and see).
I have just returned from Arctrade Euro where I have acquired a set of thread measuring wires so I will have a look at them this evening and see what I can learn.

Lew_Merrick_PE:
Terry,

Other than the fact that I have a (relatively complete) set of Whitworth taps & dies that dates to my days of driving MG's & Coopers, everything I "know" about Whitworth threads is what is published in Machinery's Handbook. If you go to http://www.scribd.com/Lew%20Merrick you will find papers on design and application data for UN (60°-V), Acme, & Stub-Acme threads. (I apologize for the mess that is now ScribD -- they were taken over by Facebook and, as soon as I can find a better spot to post, I will be closing things down there. ScribD is nearly worthless now that the social media trash has moved in.) The rule of thumb for 60°-V threads is that your clearance diameter for internal threading: Di = Major_Dia - 0.75/TPI. You would lose a bit of the minor diameter from an internal thread using that for a 55° included angle (Whitworth) nut, but that would be (about) .016 inches/inch of diameter (assuming 75% of full thread fit).

I thought the question was whether it was better to mate a (one-off, assumed) male thread to a female thread or a female thread to a male thread. In most cases, moving the set-up clear is easier using the female thread at the gauge -- and I assumed that was the actual question. If your threads have to repeatably match a designated fit standard, then making a Go/No-Go gauge using the overwire measurements is the way to go for dealing with the female threads.

minerva:
Lew,
your assumptions were correct and having read your dissertations I begin to understand the problems involved .The "Rules of Thumb" that I seek seen to depend upon so many factors, % of thread engagement , angle of thread to name but two. I have quite unwittingly made two out of the three nuts which I will use to gauge the fit on the spindle
many thanks for your input
Kindest Regards
Terry T

Lew_Merrick_PE:
Terry,

In most cases, a nominal 75% of full thread (+/-5%) is more than good enough. The factor of safety calculated (or assumed) in most designs make "gains" from increasing percentage of full thread irrelevant. I work in areas (aircraft, spacecraft, high-load vehicles, etc.) where I often need to know such things, but, as one friend of my likes to say, There's the few -- and then there's Lew...

The flip side of that lies in designing parts for mass production. Reducing the percentage of full thread increases tap or die life which reduces production costs -- an attribute that concerns few here like it does when designing something that is going to be made in multi-million units on an annual basis. I often design things that are made in 5-15 million unit annual quantities, so I think that way -- and everybody here (should) already knows that I am strange...

The history of the development of (modern) screwthreads is really quite interesting. Ernest Lowenhertz began this process circa 1750 and laid the foundation for virtually all that followed. The NAZI's tried to turn Lowenhertz into a Hero of Aryanism, so he has been written out of history as punishment. Joseph Whitworth continued (and simplified) his work. (Brian?) Sharpe carried it further (setting up the 60°-V thread) and can be argued as the creator of the system of fits we use today. The mathematical/engineering understanding of the interactions in screwthreads did not really appear until the end of the 19th century. We who have the answers pre-calculated and tabulated rarely understand the amount of work that developed such an otherwise simple mechanism. Even with all these advantages, it is still somewhat hard to grasp the minutia of a well-implemented screwthread.

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